In the field of medicine, treatments suitable for use with adult patients are not always similarly applicable for use with pediatric patients. One realm where this is particularly true is in the area of cardiopulmonary support. The field of mechanical circulatory support is evolving rapidly. However, devices that augment or replace cardiac function for adult patients cannot necessarily be used easily, or successfully, with pediatric patients, particularly with neonatal patients.
Conventional cardiopulmonary bypass (CPB) uses an extracorporeal blood circuit that is coupled between arterial and venous cannulae and includes at least a venous drainage line and an arterial return line, a blood reservoir, one pump to propel the blood in the circuit and multiple pumps to recover shed blood from the operative field and return it to the circuit via the reservoir, an oxygenator, bubble traps and or blood filters, blood flow probe, blood gas analyzers, blood transporting tubing or “lines,” ports, and valves interconnecting these components.
Traditional bypass systems utilize gravity to drain venous blood from the patient and require the use of large diameter venous cannulae and large diameter tubing in order to maintain adequate drainage. More recent techniques employ one of two methods to actively aspirate blood from the patient and augment venous return. One approach, Kinetic assisted venous drainage (KAVD), uses a centrifugal pump; and the other, Vacuum assisted venous drainage (VAVD), utilizes standard wall suction.
All of these techniques, although effective, have a particular set of limitations; including:
(1) Gravity drainage circuits have large circuit priming volumes and depend on large diameter venous cannulae thwarting its use in both pediatric and minimally invasive surgery.
(2) KAVD utilizes a centrifugal pump in the venous line that can become air-locked and ineffective during a procedure if a large amount of venous air is present. To address this limitation the use of a bubble trap in the venous line pre-pump has been described. This configuration requires that the cardiotomy reservoir or transfusion bags be attached to the venous line pre-bubble trap making the circuit much more difficult to manage. As a result KAVD systems are currently mostly utilized in adult bypass surgery when minimal blood loss is expected and the use of a cardiotomy reservoir is not necessary. Unlike adult surgery the majority of pediatric surgery takes place inside the heart and is associated with a large amount of blood loss, prohibiting the elimination of the blood reservoir.
(3) VAVD is currently the preferred CPB system of choice in pediatrics and is accomplished by connecting the venous line to a sealed hard shell cardiotomy reservoir to which a regulated vacuum source has been applied. This type of system increases air to blood interface and decreases the amount of time air bubbles have to settle out of circulation while in the reservoir, exacerbating arterial line emboli.
(4) All three of these circuits in use today are dependent on the use of multiple roller pumps to return shed blood back into the circulation from the surgical field when needed. Roller pumps require the use of tubing connecting the operative field to the blood reservoir, increasing the amount of blood held out of circulation during the procedure.
The standard technique during CPB support continues to be the use of venoarterial bypass with a membrane oxygenator, providing both hemodynamic and pulmonary support. However the current technology and design of the conventional CPB circuits are not optimal for use especially in pediatric patients. The shortcomings of the CPB circuits include large prime volumes, use of pumps that damage blood cells, extensive contact of the blood with tubing, difficulty controlling bubbles, failure to conserve blood, and difficulty due to bulky equipment.
The pumping mechanism of CPB devices pump the poorly oxygenated blood into the artificial lung where carbon dioxide is removed from the blood and oxygen is provided. Before the blood is oxygenated it can pass through a heat exchanger to raise or lower the blood temperature in order to manage patient body temperature. The oxygenated, warmed/cooled blood is then pumped back to the patient.
As noted above, disadvantages of CPB include large blood volume requirements, damage to blood cells, difficulty in managing fluid volume, increased gaseous emboli and the need to use multiple blood pumps.
Thus, the need remains for an extracorporeal blood circuit advantageously created with the pediatric or neonate patient in mind. Ideally, the size of the extracorporeal circuit would be reduced to lower the required volume to prime the circuit and to reduce the undesirable contact between the blood and the components of the circuit. In addition, the need remains for improved blood flow and blood pumping mechanisms in the extracorporeal circuits to further lessen the current problems of blood-air mixing and hemolysis of red blood cells.